Sodium sulfate treated pulp

ABSTRACT

Cellulose pulp sheets treated with sodium sulfate can be fiberized to produce sodium sulfate treated fibers that exhibit desirable densification properties. The sodium sulfate treated fibers densify to a greater degree than fibers that have not been treated with sodium sulfate.

FIELD OF THE INVENTION

The present invention relates to cellulose pulp that has been treatedwith sodium sulfate and to methods for applying sodium sulfate tocellulose pulp.

BACKGROUND OF THE INVENTION

Cellulose fibers have found widespread application in absorbentarticles, such as diapers and feminine hygiene products. The cellulosefibers are generally used as an absorbent medium to acquire, transport,and hold fluids. While cellulose fibers are effective at acquiring,transporting, and holding fluids, many improvements to cellulose fibershave been made over the past decades to improve the performanceproperties of cellulose fibers in absorbent products. For example, U.S.Pat. Nos. 6,340,411 and 5,547,541 describe that webs of cellulose fiberstreated with certain polymeric and nonpolymeric materials require lesspressure to densify a web of the fibers to a given density as comparedto the pressure needed to densify a similar web of fibers without thepolymeric or nonpolymeric material present.

The cellulose fibers treated with the compositions described in U.S.Pat. No. 5,547,541 are manufactured by applying the desired compositionsto a wet laid web of cellulose fibers which has been produced, forexample, using a Fourdrinier machine. The treated wet laid web ofcellulose fibers is generally formed into a roll for bulk delivery to anabsorbent product manufacturer. The absorbent product manufacturertypically unrolls the roll and processes the web in a fiberization unitthat individualizes the fibers and prepares them for further processing.

Absorbent products including an absorbent core of superabsorbentmaterial and cellulose fibers are typically manufactured by a processthat combines cellulose fibers and superabsorbent material. In such aprocess, rolls or bales of cellulose fibers without superabsorbentmaterial are fiberized by a fiberizing apparatus such as a hammermill.These fiberized cellulose fibers are entrained in air and superabsorbentmaterial is introduced to the air entrained fibers. The air entrainedcombination of cellulose fibers and superabsorbent material is deliveredto an air lay device such as a pad former, which draws the fibers andsuperabsorbent material onto a screen and forms the fibers andsuperabsorbent material into a particular shape. These formed pads arethen removed from the pad former for further processing, includingsubjecting the formed pads to compression in order to densify the pad bydecreasing its thickness.

Reducing the thickness of the formed pads which are used in diapers isimportant to diaper manufacturers so that they can reduce the size ofpackaging which allows them to ship more diapers per volume and todisplay a larger number of diapers in a limited amount of shelf space.In addition, consumers find thinner diapers more desirable.

With this background, the present inventors have worked to address thechallenges above and have developed compositions that can be compressedto achieve articles of desirable densities and methods of providing andutilizing such compositions.

SUMMARY OF THE INVENTION

The present invention provides cellulose pulp sheets and cellulosefibers treated with sodium sulfate that are useful in absorbent coresformed from the treated pulp or fibers. The compositions of the presentinvention can be formed into absorbent articles for absorbing fluidssuch as aqueous fluids like urine or blood. The compositions are usefulin absorbent articles such as diapers, incontinent devices and femininehygiene products. The compositions of the present invention can becompressed to densities that manufacturers of absorbent articles shouldfind desirable.

In one aspect, the present invention relates to a cellulose pulp sheetthat includes cellulose fibers, water and sodium sulfate applied to thecellulose fibers in an amount ranging from about 0.1 to 15 weightpercent based on dry fiber weight. The cellulose pulp sheet can befiberized into individualized fibers, laid into a pad, and thencompressed. The invention also relates to a method for producing acellulose pulp sheet which includes the steps of providing cellulosepulp, forming a cellulose pulp sheet from the cellulose pulp, andapplying sodium sulfate to the cellulose pulp sheet.

In another aspect, the present invention relates to a method forproducing a densified web of cellulose fibers that includes the step ofproviding cellulose fibers treated with sodium sulfate. The treatedcellulose fibers are fiberized and formed into a web. The web iscompressed to form a densified web.

In yet another aspect, the present invention relates to fibers that havebeen treated with sodium sulfate. The cellulose fiber before treatmentwith sodium sulfate exhibit a first density after application andrelease of a compression load. The treated fibers include water andsodium sulfate. The cellulose fibers after treatment with sodium sulfatedensify to a second density after application and release of thecompression load. The sodium sulfate is present in an amount so that thesecond density is at least 5 percent greater than the first density.

The sulfate treated fibers of the present invention may be furthertreated with an oil in order to provide fibers that retain materialssuch as superabsorbent materials within a web of the fibers.

Manufacturers of absorbent articles will find the sodium sulfate treatedpulp and fibers of the present invention useful in their absorbentproducts due to the densification properties of cellulose pulp fiberstreated in accordance with the present invention. The methods of thepresent invention provide suitable means for producing the treatedcellulose pulp fibers that exhibit densification properties thatabsorbent article manufacturers should find desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a graph illustrating the results of compression testing todetermine the densification properties of absorbent structurescontaining cellulose fibers treated in accordance with the presentinvention;

FIG. 2 is a graph illustrating the results of compression testing todetermine the densification properties of structures containingcellulose fibers treated in accordance with the present invention; and

FIG. 3 is a schematic illustration of a wet laid web manufacturing lineillustrating the application of sodium sulfate to a wet laid web ofcellulose fibers in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As used herein, the term “fiber” refers to natural or synthetic fibers.Such fibers may be physically pretreated, e.g., by subjecting the fibersto steam, or chemically treated, e.g., by crosslinking the fibers. Thefibers may also be twisted or crimped as desired.

A particular type of fiber are cellulose fibers. A particular example ofa cellulose fiber is wood pulp fiber. Wood pulp fibers can be hardwoodpulp fibers or softwood pulp fibers. The cellulose pulp fibers may bechemical, thermomechanical, chemithermomechanical or combinationsthereof. Such wood pulp fibers can be obtained from well known chemicalprocesses such as the kraft or sulfite processes. Other cellulose fibersinclude lyocell, linen, chopped silk fibers, bagasse, hemp, jute, rice,wheat, bamboo, corn, sisal, cotton, flax, kenaf, peat moss, and mixturesthereof. When the fibers are cellulose fibers, they may be pretreatedwith chemicals to result in lignin or cellulose-rich fiber surfaces. Inaddition, the fibers may be bleached.

Examples of synthetic fibers include acrylic, polyester, carboxylatedpolyolefin, and polyamine fibers.

Sodium sulfate (Na₂SO₄) is available in the form of white crystals orpowder from numerous commercial sources.

As used herein, the term “superabsorbent material” refers to polymersthat swell on exposure to water and form a hydrated gel (hydrogel) byabsorbing large amounts of water. Superabsorbent materials exhibit theability to absorb large quantities of liquid, i.e., in excess of 10 to15 parts of liquid per part thereof. These superabsorbent materialsgenerally fall into three classes, namely starch graft copolymers,crosslinked carboxymethylcellulose derivatives and modified hydrophilicpolyacrylates. Examples of such absorbent polymers are hydrolyzedstarch-acrylonitrile graft copolymer, a neutralized starch-acrylic acidgraft copolymer, a saponified acrylic acid ester-vinyl acetatecopolymer, a hydrolyzed acrylonitrile copolymer or acrylamide copolymer,a modified crosslinked polyvinyl alcohol, a neutralizedself-crosslinking polyacrylic acid, a crosslinked polyacrylate salt,carboxylated cellulose, and a neutralized crosslinked isobutylene-maleicanhydride copolymer.

Superabsorbent particles are available commercially, for example starchgraft polyacrylate hydrogel fines (IM 1000F) from Hoechst-Celanese ofPortsmouth, Va., or larger particles such as granules. Othersuperabsorbent particles are marketed under the trademarks SANWET(supplied by Sanyo Kasei Kogyo Kabushiki Kaisha), SUMIKA GEL (suppliedby Sumitomo Kagaku Kabushiki Kaisha and which is emulsion polymerizedand spherical as opposed to solution polymerized ground particles),FAVOR (supplied by Stockhausen of Greensboro, N.C.), and NORSOCRYL(supplied by Atochem).

The term oil as used generally applies to a wide range of substances.Oils may be derived from animals or from plant seeds or nuts, and thesetypes of oils tend to be chemically identical with fats, with the onlydifference being one of consistency at room temperature. Animal andplant oils are composed largely of triglycerides of the fatty acids,oleic, palmitic, stearic, and linolenic acid. Oils may also be derivedfrom petroleum sources. Petroleum-based oils generally include a mixtureof hydrocarbons. As used herein, the term “oil” refers to oils that havemelting points below the temperature at which the oil is applied to thefibers as described below in more detail. Such temperature willgenerally be below 25° C., but could be higher. If the melting point ofthe oil is greater than the ambient temperature at which the oil isapplied to the fibers, the oil can be heated to liquefy it. This ensuresthat the oils remain liquid during their application to the fibers. Oilsuseful in the present invention should also have a vapor pressuresufficiently low to prevent evaporation either during their applicationor during use.

The oil should not penetrate the walls of the fibers so rapidly that itbecomes unavailable to retain the superabsorbent material whensuperabsorbent material is contacted with the oil treated fibers. Theoil preferably resides on the surface of the fibers during the usefullife of the absorbent article made from the fibers. To that end, oils ofhigher molecular weight penetrate the fiber wall more slowly than oilsof a lower molecular weight.

Examples of “oils” as that term is used herein include fats and theircomponent fatty acids. As described above, fats are naturally occurringesters of long chain carboxylic acids and the triol glycerol. Theseesters are also referred to as triglycerides. The hydrolysis of fatsyields glycerol and three component carboxylic acids. These straightchain carboxylic acids which may be obtained from the hydrolysis of fatsare called fatty acids and include one carboxylic acid group. Fattyacids may be saturated or unsaturated. The most common saturated fattyacids are lauric acid, myristic acid, palmitic acid, and stearic acid.Other fatty acids include oleic acid, linoleic acid, and linolenic acid.Generally, the melting point of a fat depends on the amount ofunsaturation in the fatty acids. Fats with a preponderance ofunsaturated fatty acids generally have melting points below about 25° C.Specific examples of oils as that term is used herein include soybeanoil, cottonseed oil, linseed oil, tung oil, castor oil, coconut oil,olive oil, canola oil, safflower oil, corn oil or jojoba oil. Jojoba oilis a light yellow liquid at room temperature that is not technically anoil or fat, but rather is a wax. A wax is an ester of fatty acids withlong chain monohydric alcohols. The term oil as used herein is intendedto include jojoba oil and other waxes that are liquid at temperaturesthat they are applied to fibers. It should be understood that theforegoing is a list of exemplary oils and that oils useful with thesodium sulfate treated fibers of the present invention are notnecessarily limited to the foregoing oils. It should be understood thatuse of the term “oil” in this application refers not only to the oilitself comprising a mixture of various fat and fatty acid components,but also includes the individual isolated fats, and the isolated fattyacids that result when the fats are hydrolyzed. For example, the term“oil” as used herein also refers to the fatty acids oleic, palmitic,stearic, and linolenic, that form the most common triglycerides in manyoils derived from animals and plants and would be useful to retainsuperabsorbent material in an absorbent structure comprising oil-treatedfibers and superabsorbent material.

The term “oil” as used herein also refers to unsubstituted alkanes,alkenes, alkynes, cycloalkanes, cycloalkenes, cycloalkynes, aromatics,and mixtures thereof derived from petroleum or animal sources that havemelting points below the temperature at which the oil is applied to thefibers, e.g., about 25° C. Such oils are generally derived frompetroleum sources, but may also be derived from animal sources. Oils ofthis type should have vapor pressure sufficiently low to preventevaporation of the oil during application or use. Specific examples ofthese types of oils include mineral oil, paraffin oil, hexadecane,squalane, and squalene.

As used herein, mineral oil is an example of a highly refined liquidpetroleum derivative. Mineral oil is light, clear, colorless, andodorless and is also referred to as medicinal oil. Mineral oil is usedmedicinally as an internal lubricant and for the manufacture of salvesand ointments.

Paraffin oil is an example of an oil that is either pressed or drydistilled from paraffin distillate obtained from the distillation ofpetroleum.

Squalane is an example of an alkane derived from animal sources, such asthe sebum. Squalene is an example of an alkene; more specifically, aterpene derived from animal sources, such as the human sebum or sharkliver oil. Squalene may also be isolated from oils derived from plants,such as olive oil, wheat germ oil, rice bran oil, and yeast.

In accordance with the present invention, the amount of sodium sulfateadded to the cellulose pulp sheet can vary over a wide range. Amounts ofsodium sulfate solids in the treated cellulose sheets can range fromabout 0.1 wt. % to 15 wt. % based on the dry fiber. weight. A narrowerrange of amounts is about 1.0 to 10.0 wt. % based on dry fiber weight,and an even narrower range is about 1 to 7 wt %. These amounts of sodiumsulfate can be provided in the treated cellulose sheets by applying anaqueous solution of sodium sulfate. The amount of sodium sulfate in theaqueous solution can vary over a wide range. Preferably, the amount ofsodium sulfate in the aqueous solution that is applied to the cellulosepulp sheet is chosen such that a desired level of loading of sodiumsulfate solids onto the cellulose pulp sheet is achieved without thewater content of the cellulose fiber sheet rising above about 15 to 20wt. % based on the total product weight. If the water content of thepulp sheet is excessive, the pulp sheet is difficult to fiberize and maybe susceptible to premature degradation, such as from mold growth orrotting. In certain embodiments, when the aqueous solution of sodiumsulfate is applied to the pulp sheet before the pulp sheet is dried,larger amounts of water can be introduced into the pulp sheet providedthat the subsequent drying steps reduce the water content of the pulpsheet to a level below about 20 wt. % based on the total product weight.Sufficient amounts of sodium sulfate solids should be added to thecellulose pulp sheet so that when the cellulose pulp sheet is fiberizedthe resulting fibers exhibit densification properties that are superiorto the densification properties of fibers that have not been treatedwith sodium sulfate.

Aqueous solutions containing from 5 to 33 wt. % sodium sulfate areuseful in accordance with the present invention. Aqueous solutionscontaining from about 5 to 30 wt. % or about 5 to 27 wt. % sodiumsulfate are preferred because it has been observed that the percentincrease in the density of the fibers when the aqueous solution containsabout 25 wt. % sodium sulfate is greater than when the aqueous solutioncontains about 33 wt. % sodium sulfate.

The sodium sulfate solution can be applied to the cellulose pulp sheetin a number of different ways. The present invention is not limited toany particular application technique. Examples of suitable applicationtechniques include spraying, rolling, dipping, and the like. Thesolution can be applied to one or both sides of the cellulose pulpsheet. Alternatively, the solution can be applied to fibers that are notin sheet form, e.g., individualized fibers. The solution can be heatedprior to its application, although this is not required. Alternatively,the cellulose pulp sheet can be at a temperature above room temperaturewhen the sodium sulfate solution is applied. In view of the decreasingsolubility of sodium sulfate in water as the temperature of the solutiondecreases, in certain embodiments, particularly those where theconcentration of the sodium sulfate in the solution is near itssolubility limit, it is advantageous to preheat the solution or the pulpsheet in order to reduce crystallization of sodium sulfate from thesolution during or right after its application. Heating the aqueoussolution of sodium sulfate or heating the cellulose pulp sheet prior toapplication of the sodium sulfate solution is one means for introducingmore sodium sulfate into the web.

As described above, when the sodium sulfate treated fibers are subjectedto and released from a compression load, they densify to a density thatis higher than the density that is achieved when fibers that have notbeen treated with sodium sulfate are subjected to the same compressionloading and releasing. In some instances, the density is increased 10%or more.

As described above, oil can be applied to the sodium sulfate treatedfiber. The particular way that oil is applied to the fibers is notcritical. Examples of techniques for applying oil to the fibers includethe use of a gravure-type roll coater to coat a web of the fibers.Alternatively, oil can be sprayed onto a web of the fibers or the fiberscan be immersed in a bath of oil. The oil may also be added to thefibers as a web of the fibers is being broken up, such as in ahammermill. The amount of oil applied to the fibers should be sufficientto achieve retention of superabsorbent material, but not so much as tohave a significant adverse affect on the fluid absorption properties ofthe fibers, such as the fluid acquisition rate or the amount of fluidabsorbed by a web of the fibers. Manufacturers of absorbent articlesthat include absorbent structures containing oil-treated fibers desirethat the fluid absorption properties of such structures be similar to orsuperior to the fluid absorption properties of the absorbent structuresthat the manufacturer is considering replacing. Ideally, the absorbentstructures would exhibit fluid acquisition properties that are at leastas desirable as the fluid acquisition properties of similar absorbentstructures manufactured from fibers that have not been treated with oil.The amount of oil applied to the fibers should also not be so great thatit adversely impacts the fiberization of the web of oil-treated fibers.Suitable amounts of oil applied to the fibers include about 0.5 wt. % toabout 20 wt. % oil based on the weight of oven dried fibers. A narrowerrange is 1.0 wt. % to about 15 wt. % oil based on the weight of ovendried fibers and an even narrower range is 1.0 wt. % to about 10 wt. %oil based on the weight of oven dried fibers.

The form of the fibers to which the oil is applied can vary. If a rollcoater is used, the fibers can be in the form of a sheet of fibers. Forexample, the oil can be applied to a wet laid sheet of fibers having abasis weight of at least 350 grams per meter² and a density of at leastabout 400 kg/meter³.

The oil may be added neat, or it may be diluted with solvent thatevaporates after application of the oil to the fibers. The solventshould not adversely affect the attachment of superabsorbent material tothe fibers or the fluid acquisition and fluid retention properties of anabsorbent article that contains the oil treated fibers.

FIG. 3 illustrates a wet laid sheet manufacturing line such as a woodcellulose pulp sheet manufacturing line 10. In this manufacturing line,a pulp slurry 12 is delivered from a headbox 14 through a slice 16 andonto a Fourdrinier wire 18. The pulp slurry 12 typically includes woodpulp fibers and may also include synthetic or other non-cellulose fibersas part of the slurry. Water is drawn from the pulp deposited on wire 18by a conventional vacuum system, not shown, leaving a deposited pulpsheet 20 which is carried through a dewatering station 22, illustratedin this case as two sets of calendar rolls 24, 26 each defining arespective nip through which the pulp sheet or mat 20 passes. From thedewatering station, the pulp sheet 20 enters a drying section 30. In aconventional pulp sheet manufacturing line, drying section 30 mayinclude multiple canister dryers with the pulp mat 20 following aserpentine path around the respective canister dryers and emerging as adried sheet or mat 32 from the outlet of the drying section 30. Otheralternate drying mechanisms, alone or in addition to canister dryers,may be included in the drying stage 30. The dried pulp sheet 32 has amaximum moisture content pursuant to the manufacturer's specifications.Typically, the maximum moisture content is no more than 10% by weight ofthe fibers and most preferably no more than about 6% to 8% by weight.Unless overly damp fibers are immediately used these fibers are subjectto degradation by, for example, mold or the like. The dried sheet 32 istaken up on a roll 40 for transportation to a remote location, that is,one separate from the pulp sheet manufacturing line, such as at a user'splant for use in manufacturing products. The dried pulp sheets have abasis weight of about 200 g/m² to about 1000 g/m² or more and a densityon the order of at least about 0.5 g/cm³ to about 1.2 g/cm³. Dried pulpsheets having the foregoing basis weights are structurally distinct formlighter basis weight sheets of wet laid or airlaid wood pulp fibers suchas tissue paper, paper towels, or other types of paper-like wet laid orairlaid webs of cellulose fibers. Alternatively, the dried sheet 32 iscollected in a baling apparatus 42 from which bales of the pulp 44 areobtained for transport to a remote location.

The sodium sulfate solution can be applied to the pulp sheet from one ormore applying devices, one of which is indicated at 50 in FIG. 3. Anyapplying device may be used, such as streamers, sprayers, roll coaters,curtain coaters, immersion applicators, or the like. Sprayers aretypically easier to utilize and incorporate into a pulp-sheetmanufacturing line. As indicated by the arrows 52, 54, and 56, thesodium sulfate may be applied at various locations or at multiplelocations on the pulp sheet manufacturing line, such as ahead of thedrying stage 30 (indicated by line 52), intermediate the drying stage 30(as indicated by line 54), or downstream from the drying stage 30 (asindicated by the line 56). At location 52, the water remaining in thesheet or mat 20 at this stage tends to interfere with the penetration ofthe materials into the sheet. Consequently, application of the sodiumsulfate solution after some drying has taken place, for example atlocation 54, is preferable. If the sodium sulfate solution is applied atlocation 56 in an amount which would cause the moisture content of thesheet to exceed the desired maximum level, an additional drying stage(not shown) may be included in the pulp manufacturing line to bring themoisture content down to the desired level.

The oil can be applied to the pulp sheet from the same types of devicesand locations as described above with respect to the sodium sulfatesolution.

The rolls 40 or bales 44 of the treated wet laid web of fibers may betransported to a remote location for use by a user. These rolls or balesare then refiberized by a fiberizing device, such as a hammermill whichmay be used alone or in conjunction with other devices such as pickerrolls or the like for breaking up the sheet 32 or bales 42 intoindividual fibers. Depending on the end use, the individualized fibersmay be combined with particulate material, such as superabsorbentparticles, and/or airlaid into a web and densified.

With this approach, the end user of the treated fibers may readilyselect particles to be combined with the fibers. The user hasflexibility in air laying or otherwise processing the treated fibers ofthe present invention into a finished product.

The treated fibers and superabsorbent material can be combined and thenformed into an absorbent structure in the following manner. Rolls orbales of treated fibers, without particles, are fiberized by afiberizing device such as a hammermill. The individualized fibers areair entrained during which time the superabsorbent material can be addedthereto. The air entrained fibers and superabsorbent material are thendelivered to an air laying device, such as a pocket former, and formedinto a desired shape. The formed pad is removed from the air layingdevice for further processing, including subjecting the pad to acompression load to reduce the thickness of the pad and increase itsdensity. The formed pads are in the form of a web or mass of fibers usedas absorbent structures in absorbent articles such as the ones discussedabove.

It should be understood that in an alternative embodiment, the sodiumsulfate solution and oil can be applied to the fibers while they are airentrained.

As illustrated in the examples that follow, fibers treated with a sodiumsulfate solution in accordance with the present invention exhibitdesirable densification properties.

The following examples are intended to illustrate certain embodiments ofthe present invention and are not intended to limit the scope of thepresent invention.

EXAMPLE 1 Preparation of Sodium Sulfate Treated Cellulose Pulp

Southern Pine wood cellulose pulp sheet available from WeyerhaeuserCompany under the designation NB 416 from New Bern, N.C. with a startingmoisture content of 6% by weight (based on total sheet weight) wasbrought to a temperature of 120-140° F. by storing in a zippered plasticbag in an oven. The pulp sheet was then quickly removed from the bag andcoated in a Black Brothers gravure-type roll coater with a solution ofsodium sulfate. The gravure coater results in the application of auniform coating of the sodium sulfate solution over one entire surfaceof the pulp sheet from where it is rapidly soaked up by the sheet. Thesodium sulfate was obtained from Sigma-Aldrich (CAS number 7757-82-6anhydrous sodium sulfate, 99% reagent grade). The sodium sulfatesolution had a solids content of 24.8% with the balance being water. Thesodium sulfate is dissolved in water at 33° C. This 24.8 wt. % solutionwas applied to the wood pulp sheet at a rate of 10.5 parts by weightsolution to 100 parts by weight of pulp sheet, resulting in a loading ofactive (dry basis) sodium sulfate solids of ˜2.8 wt. % based on the dryfiber content of the pulp sheet. The final total moisture content of thewood pulp cellulose sheet treated with sodium sulfate is ˜12.6 wt. %based on the total final product weight.

The treated sheet was stored in a plastic zippered bag for 24 hours atroom temperature to allow the added moisture to migrate and reachequilibrium within the whole sheet. The sheet was then fiberized in alaboratory hammermill and the resultant fluff was fed to a rotary pocketformer and resulted in fluff pads measuring ˜12″×5″ with a basis weightof ˜300 grams per square meter (gsm). The pads were placed in zipperedplastic bags to preserve moisture until used in the densification testbelow.

After removal from the bags, the rectangular 12″×5″ pads were cut intosmaller square pads measuring 10 cm×10 cm using a die. The 10×10 cm padswere densified in a hydraulic flat press under loads of either 50 psi,100 psi, and 150 psi. The pressure was only held momentarily and thenreleased. Different pads were used for each of the successively higherloads. Caliper (thickness) of the pads was determined using a calipergauge with a wide “foot” designed to apply only moderate pressure to thepad of ˜0.2 psi (i.e., it does not materially densify the pad in the actof determining caliper). The densities of the pads were calculated fromthe caliper and basis weight measurements.

Results of the density measurements versus applied pressure are shown inFIG. 1 and show that the sodium sulfate treated pulp (2.8% loading from24.8 wt. % solution) attains a higher density (up to about 14%) for agiven pressure compared to the untreated NB416 pulp having a moisturecontent of 6 wt. % which was incorporated as a control.

EXAMPLE 2 Preparation of Sodium Sulfate Treated Cellulose Pulp

Example 2 was identical to Example 1 in every respect except that the24.8 wt. % solution of sodium sulfate was applied to the wood pulp sheetat a rate of 21.6 parts by weight solution to 100 parts by weight ofpulp sheet, resulting in a loading of active on a (dry basis) sodiumsulfate solids of ˜5.7 wt. % based on the dry fiber content of the pulpsheet. The final total moisture content of the wood pulp cellulose sheettreated with sodium sulfate is ˜18.3 wt. % based on the total finalproduct weight.

Again, results of the density measurements versus applied pressure areshown in FIG. 1 and show that the sodium sulfate treated pulp (5.7%loading from 24.8 wt. % solution) attains a higher density (up to about25.7%) for a given pressure compared to the untreated NB416 pulp havinga moisture content of 6 wt. % which was incorporated as a control. Thedata also shows that the higher loading of sodium sulfate at higherfinal moisture content is advantageous.

EXAMPLE 3 Preparation of Sodium Sulfate Treated Cellulose Pulp

Several air dry (˜6% moisture content) 4″ width strips of NB416 pulpsheet weighing about 40 g were brought to a temperature of 120-140° F.by placing in zippered plastic bags in an oven. The strips were quicklyremoved from the oven and, whilst hot, were treated with 5.7 g of a 33wt. % solution of sodium sulfate solution that had been preheated to 33°C. Application of the solution was made to one side of the wood pulpsheet using a syringe. Liquid was applied in lines along the full lengthof the machine (long) direction of the paper and were spaced apart byapprox. ¼″. This resulted in a loading of active (on a dry basis) sodiumsulfate solids of ˜5.0 wt. % based on the dry fiber content of the pulpsheet. The final total moisture content of the wood pulp cellulose sheettreated with sodium sulfate is ˜13.61 wt. % based on the total finalproduct weight. Control strips of NB416 having a moisture content of 6wt. % (no sodium sulfate addition) were also included in thefiberization and densification procedures that follow below.

The pulp strips thus treated are placed in zippered plastic bags for 24hours and then removed and fiberized using a laboratory Fitz hammermill.Resultant fluff was stored for about 16 hours in a room at 50% RH (inbags with the tops open) and then is formed (using a laboratory padformer) into 6″ diameter round pads of ˜300 gsm basis weight from which10 cm×10 cm square pads are cut and subject to the densificationprocedure as described in Example 1. Density of the resultant pads isshown in FIG. 2.

Results illustrated in FIG. 2 show that the sodium sulfate treated pulp(5.0% loading from 33 wt % solution) attains a higher density for agiven pressure compared to the untreated NB416 pulp which wasincorporated as a control. However, in comparison with FIG. 1 it isclear that the level of density increase over the control is reducedwhen using a 33% sodium sulfate solution versus the 24.8% solution. Inthe case of the 33% solution (at 5% chemical add-on) the increase at 150psi is ˜4% higher than the control (0.17 g/cc vs. 0.163 g/cc) whereas inthe case of the 24.8% solution of Example 2 (at a similar 5.7% chemicaladd-on) the density at 150 psi is 25.7% higher than the control (0.176g/cc vs. 0.14 g/cc). It should be pointed out that in these types ofexperiments the density that the control pulp attains variesconsiderably with the type of fiberizer, pad former and ambient humidityconditions. Given this, each experiment had its own internallyconsistent control as the basis for judging the performance of treatedpulps.

1. A cellulose pulp sheet comprising: cellulose fibers; water; andsodium sulfate applied to the pulp sheet in an amount ranging from about0.1 to 15 weight percent based on dry fiber weight.
 2. The cellulosepulp sheet of claim 1, wherein the sodium sulfate is applied to the pulpsheet in an amount ranging from about 1 to 10 weight percent based ondry fiber weight.
 3. The cellulose pulp sheet of claim 1, wherein thecellulose fibers are wood pulp fibers.
 4. The cellulose pulp sheet ofclaim 2, wherein the sodium sulfate is applied to the pulp sheet as anaqueous solution containing about 5 to 33 weight percent sodium sulfatesolids.
 5. The cellulose pulp sheet of claim 4, wherein the sodiumsulfate is applied to the pulp sheet as an aqueous solution containingabout 5 to 30 weight percent sodium sulfate solids.
 6. The cellulosepulp sheet of claim 5, wherein the sodium sulfate is applied to the pulpsheet as an aqueous solution containing about 5 to 27 weight percentsodium sulfate solids.
 7. The cellulose pulp sheet of claim 1, whereinthe water content is less than about 20 weight percent based on totalproduct weight.
 8. The cellulose pulp sheet of claim 7, wherein thewater content is less than about 15 weight percent based on totalproduct weight.
 9. The cellulose pulp sheet of claim 1, furthercomprising an oil.
 10. Treated fibers comprising: cellulose fibers, thecellulose fibers after application and release of a compression loadbeing densified to a first density; water; and sodium sulfate, thecellulose fibers after treatment with the sodium sulfate densifying to asecond density after application and release of the compression load,the sodium sulfate being present in an amount effective to result in thesecond density being greater than the first density.
 11. The treatedfibers of claim 10, wherein the second density is at least 5 percentgreater than the first density.
 12. The treated fibers of claim 11,wherein the sodium sulfate is present in an amount greater than about1.0 wt. % based on dry fiber weight.
 13. The treated fibers of claim 10,further comprising an oil.
 14. A method for producing a cellulose pulpsheet comprising providing cellulose pulp; forming a cellulose pulpsheet from the cellulose pulp; and applying sodium sulfate to thecellulose pulp sheet.
 15. The method of claim 14, wherein the cellulosepulp is wood pulp.
 16. The method of claim 14, wherein the sodiumsulfate is applied to the cellulose pulp sheet in an amount that resultsin a loading of about 0.1 to 15 weight percent based on dry pulp. 17.The method of claim 14, wherein the sodium sulfate is applied to thecellulose pulp sheet in an amount that results in a loading of about 1.0to 10 weight percent based on dry pulp.
 18. The method of claim 14,wherein the sodium sulfate is applied as an aqueous solution containingfrom about 5 to 33 weight percent sodium sulfate solids.
 19. The methodof claim 14, wherein the cellulose pulp sheet further comprises waterand the water content of the cellulose pulp sheet after application ofthe sodium sulfate is less than about 20 weight percent based on totalproduct weight.
 20. The method of claim 14, wherein the temperature ofthe cellulose pulp sheet when the sodium sulfate is applied is at least25 degrees Celsius.
 21. A method for producing a densified web ofcellulose fibers comprising: providing cellulose fibers treated withsodium sulfate; fiberizing the cellulose fibers treated with sodiumsulfate; forming the fiberized cellulose fibers treated with sodiumsulfate into a web; and compressing the web.
 22. The method claim 21,wherein the cellulose fibers are wood pulp fibers.
 23. The method ofclaim 21, wherein the sodium sulfate is present on the cellulose fibersin an amount ranging from about 0.1 to 15 weight percent based on dryfibers.